NG, you've already determined that it wasn't the mainspring so there are five other possibilities that I can think of. Bent firing pin, obstruction or dirty firing pin hole, something dragging on the hammer slowing its travel, compare the rear diameter of the firing pin with another and see if it might have been peaned. I've seen them pean enough to actually get stuck in the firing pin stop. The last possibility would be excess headspace. One more; slide not returning fully to battery. George